1. Field of the Invention
The present invention relates to an endless belt member which is rotationally moved as being tensioned by a plurality of tension members and an image forming apparatus including the belt member.
2. Description of the Related Art
There has been proposed a variety of image forming apparatuses to transfer a toner image by utilizing an endless belt member which is rotationally moved as being tensioned by a plurality of tension members. For example, an image forming apparatus in which a toner image is primarily transferred from a photosensitive drum to a transfer belt and the toner image borne at the transfer belt is secondarily transferred to a recording material is known. Further, an image forming apparatus in which a toner image is transferred from a photosensitive drum to a recording material borne and conveyed by a transfer belt is known.
In the abovementioned image forming apparatuses, various electrical external force or mechanical external force such as transferring of a toner image, cleaning after transferring and neutralizing, are applied to either transfer belt. Accordingly, the abovementioned transfer belt requires a variety of characteristics such as durability against the external force, that is, mechanical strength, wearability and electrical durability.
The transfer belt is scratched by carriers contained in toner when a toner image is transferred and is slid against recording materials and abutment members such as a cleaning blade and a transfer roller. Accordingly, a surface layer of the transfer belt is required to have appropriate hardness. Meanwhile, specific endurance performance needs to be satisfied while receiving stress at the vicinities of a rib for preventing belt meandering and a tension roller edge and bending stress by the tension roller.
As a transfer belt satisfying the abovementioned wearability and durability, a transfer belt utilizing resin having excellent mechanical strength of a high Young's modulus such as polyimide has been disclosed in Japanese Patent Application Laid-Open No. 2001-047451. However, with the transfer belt disclosed in Japanese Patent Application Laid-Open No. 2001-047451, there has been a problem of cost increase since the material itself is expensive. When a transfer belt is made of an inexpensive resin material, there has been a problem in durability such as wearability. Then, in Japanese Patent Application Laid-Open No. 2000-56585 and Japanese Patent Application Laid-Open No. 2006-184361, both wearability and durability have been intended to be satisfied by the arrangement of a surface layer having a higher Young's modulus than that of a base layer at the outer circumference of the base layer which is formed of an inexpensive resin material. However, there may be a case in which stress is locally concentrated at the rib for preventing meandering to prevent the belt from meandering, and then, the belt is deformed at the vicinity of the rib where the stress is concentrated. Accordingly, when the surface layer is arranged on the entire face of the belt, peeling and lifting of the surface layer occur due to the deformation of the belt, so that a problem of functionally harmful effects arises.
To address the above issues, the present invention further suppresses peeling and lifting of a surface layer of a transfer belt caused by local stress concentration at a rib for preventing meandering arranged at the transfer belt while being less expensive and having sufficient wearability and durability against external force.